Patent Literatures 1 and 2 disclose known capacity virtualization techniques. According to these techniques, a storage apparatus manages a virtual volume composed of a plurality of virtual areas and a pool composed of a plurality of real areas. When the storage apparatus receives a write command that specifies an address belonging to the virtual area, the storage apparatus allocates an unallocated real area to a write-destination virtual area and writes data involved with the write command to the real area.
Patent Literature 3 discloses a technique that allows each of a plurality of storage apparatuses (modules) to use a pool in another storage apparatus.